Materials used in the high temperature sections of modern gas turbine engines and other similar machines require an optimized combination of several properties, including mechanical properties as well as resistance to environmental degradation (oxidation and corrosion). Superalloys, based on nickel, cobalt, or iron, often possess these desired properties and have found widespread use in industry. The term "superalloys" is used to denote the class of refractory modified nickel, cobalt or iron based metal alloys specifically developed for high temperature service. Generally speaking, superalloys with the highest temperature capability have a single crystal microstructure. In other words, the casting is of a single grain, and has no internal grain boundaries. Single crystal castings are described in U.S. Pat. No. 4,209,348 to Duhl et al., the contents of which are incorporated herein by reference. The Duhl et al. patent describes a single crystal superalloy casting having excellent mechanical properties and oxidation resistance.
Many superalloys owe their oxidation resistance to their ability to form a protective oxide film on the casting surface during use at elevated temperatures. The oxide film must be adherent in order for it to provide long term oxidation resistance. The most protective films are generally considered to be aluminum oxide.
The element sulfur has been shown to detrimentally affect the ability of oxide films to adhere to the casting surface. Accordingly, a significant effort has been directed into removing sulfur from the casting or immobilizing sulfur in the casting. However, the prior art methods for addressing the sulfur problem have not been entirely successful. Accordingly, what is still needed in the superalloy industry is a method for dealing with sulfur and for making single crystal superalloy castings having the required levels of mechanical strength and oxidation resistance.